The present invention relates to an acoustic imaging apparatus which exploits high-frequency acoustic energy. More particularly, it relates to an acoustic microscope of the reflection type in which a focused ultrasonic beam is projected on a sample so as to obtain an image on the basis of the resulting reflected acoustic waves.
A scanning acoustic microscope has been proposed and studied as an apparatus for obtaining an image expressive of the microscopic elastic properties of an object to-be-observed by utilizing a hypersonic wave which has an acoustic frequency of 1 GHz and accordingly exhibits an acoustic wavelength of approximately 1.5 .mu.m in the water.
In an example disclosed in U.S. Pat. No. 4,028,933, an acoustic lens whose F number is small is employed for projecting an ultrasonic beam which is very sharply focused on a sample to-be-observed through a propagating medium such as water. On the other hand, while the sample is being scanned mechanically, perturbed energy from the sample is detected, and the detected acoustic waves are displayed on a CRT screen. Thus, microscopic images are obtained.
Setups for detecting the perturbed energy from the sample as described above are classified into two types; the transmission type and the reflection type. In the former, acoustic waves having passed through the sample are detected to obtain an image. Therefore, a transmitting acoustic lens or a transmitting concave transducer and a receiving acoustic lens or a receiving concave transducer are arranged so as to oppose to each other with the sample interposed therebetween. Since the two, transmitting and receiving lenses or concave transducers must be arranged in confocal fashion, adjustments for the alignment thereof become complicated and very subtle. Meanwhile, in the latter, acoustic waves reflected from the sample are detected, so that a single acoustic lens or concave transducer can be used for both the transmission and reception of waves. This brings forth the advantages that the construction of the apparatus is simplified and that the adjustments for the alignment are dispensed with.
A biological tissue which is an important object to be imaged by the acoustic microscope has heretofore been observed with the transmission type apparatus, not with the reflection type apparatus. Concretely, water or a physiological salt solution is used as a propagating medium and is held between the opposing transducers. The sample is supported within this medium in the state in which it is stuck to a film of a thickness and a material that permit the presence thereof to be neglected in propagating the acoustic waves from the medium to the sample.
The reason why such setup of the transmission type has been employed for the observation of the biological tissue, is as follows. The biological tissue is slightly different in the acoustic impedance from the water as the propagating medium, and cannot produce an intense reflection signal. With the reflection type setup, therefore, observation at a high signal-to-noise ratio is impossible.